Does a divorce oath take effect if a person swears, unintentionally, that his friends are not in the parking lot, and they are indeed not there?

As long as you swore by divorce truthfully, then no expiation is binding upon you, and it does not count as a divorce against you, even if the matter turns out to be otherwise. It is advisable to avoid swearing by divorce and to limit oaths to swearing by Allah Almighty.